JPH09130146A - Low frequency data modulation circuit for fm radio unit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the current consumption and also to secure an automatic frequency control function for the modulation circuit by applying the FM modulation to a voltage control oscillator oscillating by the control voltage via a modulation AF signal and using the oscillator as a reference signal source for a synthesizer of the 2nd local frequency and the reference frequency. SOLUTION: The switches SW1 and SW2 are turned on and off respectively and at the same time the receiving signal of the receiving input Rx undergoes the automatic frequency control AFC. When this AFC is over, the lock voltage is read by an A/D converter of a control part 12. Then the SW1 and SW2 are turned off and on respectively, and the voltage read by the A/D converter is sent via a D/A converter of the part 12. The 2nd local frequency is held to be equal to the frequency that is set in a closed loop mode of a PLL synthesizer. As a result, the 2nd local frequency is equal to the mere oscillation frequency and the modulation AF signal can be modulated from DC. This output is used by the 2nd local signal and also as the reference signal of the PLL synthesizer.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low frequency data modulation circuit in an FM radio.

[0002]

2. Description of the Related Art FM radios such as car telephones have a control signal which is lower than a voice frequency band (300 Hz to 3 KHz).
The carrier wave is FM-modulated by a signal of 00 Hz or less and used as a control signal.

There are various methods for FM-modulating Tx waves with low-frequency data of 300 Hz or less in an FM radio device. First, as shown in FIG. 2, a temperature-controlled crystal oscillator (T
There is a direct method in which a modulation AF signal is input to a voltage controlled oscillator (VCO) via one PLL synthesizer including one PLLic including CXO) and a low-pass filter LPF to perform FM modulation to directly generate a Tx frequency. Second, as shown in FIG. 3, the Tx frequency is created by mixing the 1st local frequency from the Rx synthesizer with the fixed frequency created in FIG. 2 by the mixer (MIX) created by the PLL synthesizer. There is a mix method.

[0004]

However, both methods have a problem that it is difficult to design a PLL synthesizer because FM modulation is applied to a low frequency signal. On the other hand, if FM modulation is applied to the frequency of the reference oscillator TCXO with the AF signal for modulation, the design of the PLL synthesizer is easy, but it is possible to use another PLL synthesizer controlled by this reference oscillator, for example, the 1st local PLL synthesizer. Will also affect.

As shown in FIG. 4, the 1st local P
A system using two of the LL synthesizer 1 and the 2nd local PLL synthesizer 2 is also conceivable. However, in this case, it becomes difficult to design the 2nd local PLL synthesizer 2. Also in this case, the reference oscillator TCXO
Also, if FM modulation is applied with a modulating AF signal, designing a PLL synthesizer is easy, but as with the prior art, other P
There arises a problem of affecting the LL synthesizer.

From the above consideration, the reference oscillator TCVO is set to 2
It would be a solution if you had one, but in this case the cost would increase and the automatic frequency adjustment AFC would become difficult.

An object of the present invention is to provide a modulation circuit having an automatic frequency adjusting function, which solves the above problems.

[0008]

In order to solve the above problems, the present invention comprises first and second PLLs that output first and second local frequencies using a PLL synthesizer and a voltage controlled oscillator. A control unit for A / D converting the analog signal from the second PLL synthesizer and holding the converted digital signal as a lock voltage,
After the control section makes the second PLL synthesizer a closed loop, the voltage-controlled oscillator oscillated by the control voltage output by the D / A converter is FM-modulated by the AF signal for modulation. It was used as the reference signal source of the synthesizer for local frequency and reference frequency.

[0009]

1 is a circuit diagram showing an embodiment of a modulation circuit of the present invention.

The reception input 3 is connected to the first via the RF-AMP4.
Input to the mixer 5. The 1st local frequency from the PLL synthesizer 6 is input to the first mixer 5, and is input to the second mixer 7 via the bandpass filter BPF. In the second mixer 7, the voltage from the voltage controlled oscillator VCO is supplied to the signals from the PLLic 8 and the low pass filter LPF.
The modulation AF signal is input to 9 and FM-modulated 2nd
The local frequency is input. The output of the second mixer 7 is input to the intermediate frequency detection circuit 10. The AF signal output from the intermediate frequency detection circuit 10 is canceled by the cancellation circuit 11.
Is output through Here, switches SW1 and SW2 are provided in PLLic8 and LPF which configure the PLL synthesizer.
The control unit 12 is connected via. The control unit 12 is L
A / which converts analog signal from PF to digital signal
D converter and D for converting digital signals to analog signals
It has a / A converter and holds the lock voltage. The 2nd local frequency output from the voltage controlled oscillator 9 is input to the PLLic 13 via the frequency divider 1 / N, the signals from the PLLic 13 and the LPF are FM-modulated by the voltage controlled oscillator 14 by the modulation AF signal, and the reference signal is output. Becomes

When SW1 is turned on and SW2 is turned off, the second local frequency is PLLic8, LP.
PLL in loop of F, SW1, resistor R, capacitor C
Configure a synthesizer. In this state, receive input R
Automatic frequency adjustment AFC is performed on the received signal of x. AF
After the completion of C, the lock voltage is read by the A / D converter of the control unit 12, SW1 is turned off and SW2 is turned on, and the voltage read by the A / D converter is sent from the D / A converter of the control unit 12. Then, the 2nd local frequency is held so as to become the same frequency as the closed loop of the PLL synthesizer. Next, the power supply of the PLLic 8 etc. of this PLL synthesizer is turned off.

Then, the 2nd local frequency becomes a mere oscillator, and the modulation AF signal can be modulated from DC. This output is used for 2nd local signal,
Used as a reference signal for the PLL synthesizer.

In the modulation of the Tx frequency, an ideal modulator from DC is constructed by giving the modulated reference signal and, if necessary, the modulating AF signal to the voltage controlled oscillator VCO 14 as well. Since the 2nd local signal is FM-modulated by the voltage-controlled oscillator 9 with the modulating AF signal, the modulating AF signal is demodulated in the AF signal output from the intermediate frequency detection circuit IFDET.
Therefore, a cancel circuit 11 for canceling this
Is added as needed.

With the above, a call is made.
Since the d-local holds the frequency with the voltage converted by the D / A converter from the control unit 12 during the call, it is placed under the control of the AFC circuit and the control unit 12, and the call is in progress. AFC is achieved by controlling the voltage.

[0015]

As described above, according to the configuration and method of the present invention, the current consumption of the modulation circuit can be reduced and the automatic frequency adjustment function can be achieved.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a modulation circuit of the present invention.

FIG. 2 is a conventional circuit diagram.

FIG. 3 is a conventional circuit diagram.

FIG. 4 is a conventional circuit diagram.

[Explanation of symbols]

 1 1st local PLL synthesizer 2 2nd local PLL synthesizer 3 reception input 4 RF-AMP 5 1st mixer 6 PLL synthesizer 7 2nd mixer 8 PLLic 9 voltage control oscillator 10 intermediate frequency detection circuit 11 cancel circuit 12 control unit 13 PLLic 14 voltage control Oscillator

Claims (1)

[Claims] 1. A first for outputting a first and a second local frequency using a PLL synthesizer and a voltage controlled oscillator.
And a second PLL, the analog signal from the second PLL synthesizer is A / D converted, and a control unit for holding the converted digital signal as a lock voltage is provided, and the second PLL is operated by the control unit. After making the synthesizer a closed loop, FM modulation is applied to the voltage controlled oscillator oscillated by the control voltage output by the D / A converter by the AF signal for modulation, so that the second local frequency and reference frequency synthesizer A low frequency data modulation circuit in an FM radio, characterized by being used as a reference signal source.